Thinking about your post, I would have to say that JRoon's post to Kube's might be close to the answer. But I do have some other thoughts on your post.
Now, I could be wrong and willing to stand corrected, but I don't believe Amarin manufacturers anything. They buy the API and have someone encapsulate the product for them. And, we have to remember that it has been a few years now where Amarin is having to write off inventory and I might be wrong about this but I thought even had to pay API manufacturers for not being able to keep up with accepting predesignated quantities. So they may or may not be getting that much of a better deal as they used to.
Lastly, from that article, the cost to patient for GV is the covered cost. If you go on say GoodRX, even with their coupon the cost is $98. Not the $20 or so quoted in the article.